1. Field of the Invention
The present invention generally relates to electrical wiring devices commonly known as electrical receptacles which receive electrically-connecting devices known as plugs for the purpose of tapping off current for supplying numerous kinds of electrical appliances, tools and devices. More particularly, the present invention relates to grounded-type receptacles of the kind having three holes and three contacts, one of which is a ground-connected contact. Still more particularly, the present invention relates to grounding strips for such grounded-type receptacles.
2. Description of the Prior Art
A positive grounding path for grounded-type electrical receptacles has been obtained by connecting a jumper wire between the grounding lug contact, which is mounted on the receptacle, and the electrical outlet box, which is connected to earth ground via a grounded conduit. It has also been proposed to eliminate the jumper wire by using automatic grounding circuitry wherein the grounding path is obtained by electrically connecting a metal receptacle-mounting screw between a metal grounding clip attachment, which is separately connected to a grounding strip, and the metal electrical outlet box which is connected to earth ground. In this latter case, the ground connection is made from the grounded outlet box to the mounting screw and, thereupon, to the clip attachment and, in turn, to the grounding strip and, thereupon, to the grounding socket in which the ground plug is received.
All of the above-described grounding proposals generally meet current electrical code requirements; however, they have not proven to be altogether satisfactory in providing a reliable low impedance path to ground at a relatively low cost due to the fact that the electrical current must pass through a number of separate and discrete parts before it reaches earth ground. Each time the current traverses another discrete part, an undesirable voltage drop occurs across the electrical interconnection between each two successive parts, thereby resulting in a high impedance path to ground. For example, in the currently popular automatic grounding proposal described above, if the mounting screw is loosely fitted within its hole which is formed in the clip attachment, then a high resistance exists at this interconnection. Of course, the separate mountings of the clip to the grounding strip, and of the female-type grounding socket portion to the grounding strip both increase the resistance present in the grounding path. Still furthermore, the different electrical conductivities of the separate parts, due to the fact that the latter are generally constituted by different metals, likewise contributes to a poor grounding path.
Great heat and arcing might be created between any two such parts where the connection is poor due, for example, to ill-fitting parts or a poor rivet connection, or a poor soldering or weld-type connection, thereby possibly resulting in an electrical fire. An appliance plugged into this receptacle will not be provided with a good low resistance path to circuit ground, and if the casing of the appliance should be rendered electrically hot, it would pose a great safety hazard to those who are in close proximity to the appliance.
Another problem associated with prior art receptacles is that the automatic grounding portion, which underlies a mounting ear formed on the grounding strip, tends to be pulled away from the mounting ear when the mounting screw is inserted through the recess formed in the mounting ear and a juxtaposed recess formed in the automatic grounding portion. In order to eliminate such undesirable relative movement, the prior art has proposed using latches to restrain such movement; however, the restraining latches currently available have not proven to be altogether effective, particularly in cases where the installer repeatedly inserts and removes the mounting screw.
Still another drawback associated with prior art receptacles is that they do not possess sufficient mechanical strength to withstand structural damage, particularly to the grounding strip yoke. Typically, the receptacle is supported on the outlet box by a yoke which is connected at the underside of the bottom housing portion. When line cords are yanked suddenly, the receptacle may be pulled off its yoke.
Yet another drawback associated with prior art receptacles is that they are not readily convertible to isolated ground receptacles. In certain applications, typically in hospitals, it is desired to eliminate the possibility of stray currents affecting ultra-sensitive electronic equipment such as data-collecting systems, medical monitoring and analytical equipment. When equipment of this nature is grounded normally through the built-in ground, distorted signals can occur. Therefore, an additional, separate grounding wire is provided in hospitals to provide a "pure" or isolated ground path. This separate grounding path is typically run in the same raceway as the current-carrying conductors but is not a part of the normal grounding circuit. It is independent and is connected only to a suitable ground.
In order to make certain that the isolated ground path is independent from the normal grounding circuit through the outlet box, the prior art has proposed electrically-insulating material washers which surround the screw and which are placed underneath the automatic grounding portion to thereby prevent any electrical communication with the outlet box. However, such washers require separate manufacture with concomitant additional costs.